The present invention is related to the general field of sensors, and more particularly to sensors adaptable for sensing mass flow of a fluid medium, which may comprise air.
Many types of sensors for sensing mass airflow are known, and these sensors are oftentimes utilized in automotive engine control applications wherein it is cirtical to provide an accurate indication of the sensed airflow in the engine manifold. The cost of such sensors is a substantial factor. A great many prior mass airflow sensors utilize a temperature dependent resistor, or resistive film, which is exposed to the fluid medium to be sensed. Typically, these resistors, or associated circuitry, receive a power signal so that the resistors are maintained at a predetermined temperature. However, when the resistors are exposed to changes in fluid flow, the mass of the fluid results in transiently altering the temperature of the resistor which, in turn, results in altering an electrical signal, such as the electrical signal utilized to maintain the resistor at its previous steady state temperature. In this manner, an electrical signal is provided as an indication of sensed mass airflow. Sensors such as the ones described above also may utilize compensating or ambient measuring resistors to obtain more accurate mass airflow indication. A substantial concern of airflow sensors, such as those discussed above, is to have the sensor create as small a disturbance as possible in the sensed airflow such that the output signal reading will not be influenced by the sensor.
Prior mass airflow sensors such as those discussed above have been implemented by utilizing discrete hot wire resistive temperature sensing elements which must then be connected to associated sensor electronics in order to produce the desired mass airflow related electrical output signal. In some cases, film metallizations have been deposited on carrier substrates wherein these film metallizations serve as the mass airflow sensing resistive element. In such cases, the film substrates sometimes comprise sheets of thin polyimide flexible film, such as Kapton. However, in all of the prior situations additional interface connections between the actual sensing element and the remaining portion of the sensor electronics has greatly increased the cost of such sensors due to the typically extensive labor intensive operations necessary to implement reliable electrical connections between the sensor element and the sensor electronics. These connections can involve soldering fine wires between the sensor element and sensor electronics. These wires are susceptible to radio frequency interference signals. Also, providing connections from an external power source and read-out device to the sensor electronics can add to the cost of the sensor. Typically separate support structures for the sensor element and sensor electronics are required adding to the cost of the flow sensor.